(1) Field of the Invention
The present invention relates to solid-state imaging devices, and more particularly, to a solid-state imaging device including an AD converting circuit for each of columns.
(2) Description of the Related Art
Solid-state imaging devices, converting light into electric signals, are utilized in various electric appliances, such as digital video cameras, digital still cameras, and fax machines. CCD (Charge Coupled Device) image sensors and CMOS (Complementary Metal-Oxide Semiconductor) image sensors are well known as solid-state imaging devices.
A CMOS image sensor AD-converts an electric signal read from pixels arranged in a matrix, and outputs the converted electric signal, a resulting digital signal, out of the sensor.
As a conventional CMOS image sensor, there is a solid-state imaging device including an AD converting circuit for each of columns and outputting an AD converted digital signal on a line-to-line basis (See Japanese Unexamined Patent Application Publication No. 2005-303648).
A conventional solid-state imaging device, including an AD converting circuit for each of columns, shall be described hereinafter.
FIG. 1 is a block diagram exemplifying a structure of the conventional solid-state imaging device. A conventional solid-state imaging device 500 in FIG. 1 includes a pixel array 501, a column scanning unit 502, an AD converting unit 503, a reference voltage generating unit 504, a row scanning unit 505, an output unit 506, and a timing controlling unit 507.
The pixel array 501 includes pixels 508 arranged in a matrix. Each of the pixels 508 converts received light into a signal voltage, and provides the converted signal voltage to a column signal line provided on each of columns.
The column scanning unit 502 sequentially selects lines of the pixels 508.
The AD converting unit 503 dynamically converts each of signal voltages provided to associated column signal lines into a digital signal. The AD converting unit 503 includes an AD converting circuit on each column, and each of AD converting circuits includes a comparator 509, and a counter 511.
Each comparator 509 compares the signal voltage provided to the column signal line and a reference voltage RAMP, and then outputs an output signal showing the greater voltage, either the signal voltage or the reference voltage RAMP.
Using a clock ADCLK, the counter 511 counts a count value. The counter 511 suspends the counting, upon the output signal outputted from the comparator 509 inverting.
The reference voltage generating unit 504 generates the reference voltage RAMP.
The row scanning unit 505 sequentially selects columns of the associated pixels 508.
The output unit 506 outputs the digital signals converted by the AD converting unit 503 out of the conventional solid-state imaging device 500.
The timing controlling unit 507 controls operational timing of the column scanning unit 502, the AD converting unit 503, the reference voltage generating unit 504, and the row scanning unit 505.
The above structure allows the conventional solid-state imaging device 500 to AD-convert the signal voltages generated the pixels 508 on a line-to-line basis, and then to outputs the AD-converted signal voltages; namely digital signals.
The solid-state imaging device 500, however, includes as many AD converting circuits as the number of columns of the pixel 508. Thus, unfortunately, a consumption current of the AD converting unit 503 is large. Specifically, a bias current of the comparator 509 on each column is approximately 10 μA. Having approximately 2500 columns, the consumption current of the AD converting unit 503 is 25 mA. This increases the power consumption of the conventional solid-state imaging device 500. Moreover, in the case where the number of the pixels increases in the future, the power consumption of the AD converting unit 503 increases further.
In addition, a typical digital still camera and a digital video camera are battery-powered. Hence, in order to realize long recording time on digital still cameras and digital video cameras, solid-state imaging devices in the digital still cameras and the digital video cameras are desired to consume small amount of electricity.